When functioning properly, the human heart maintains its own intrinsic rhythm, and is capable of pumping adequate blood throughout the body's circulatory system. However, some people have irregular cardiac rhythms, referred to as cardiac arrhythmias. Such arrhythmias result in diminished blood circulation. One mode of treating cardiac arrhythmias uses drug therapy. Anti-arrhythmic drugs are often effective at restoring normal heart rhythms. However, drug therapy is not always effective for treating arrhythmias of certain patients. For such patients, an alternative mode of treatment is needed. One such alternative mode of treatment includes the use of a cardiac rhythm management system. Portions of such systems are often implanted in the patient and deliver therapy to the heart.
Cardiac rhythm management systems include, among other things, pacemakers, also referred to as pacers. Pacers deliver timed sequences of low energy electrical stimuli, called pace pulses, to the heart, such as via an intravascular leadwire or catheter (referred to as a "lead") having one or more electrodes disposed in or about the heart. Heart contractions are initiated in response to such pace pulses (this is referred to as "capturing" the heart). By properly timing the delivery of pace pulses, the heart can be induced to contract in proper rhythm, greatly improving its efficiency as a pump. Pacers are often used to treat patients with bradyarrhythmias, that is, hearts that beat too slowly, or irregularly.
Cardiac rhythm management systems also include cardioverters or defibrillators that are capable of delivering higher energy electrical stimuli to the heart. Defibrillators are often used to treat patients with tachyarrhythmias, that is, hearts that beat too quickly. Such too-fast heart rhythms also cause diminished blood circulation because the heart isn't allowed sufficient time to fill with blood before contracting to expel the blood. Such pumping by the heart is inefficient. A defibrillator is capable of delivering an high energy electrical stimulus that is sometimes referred to as a defibrillation countershock ("shock"). The shock interrupts the tachyarrhythmia, allowing the heart to reestablish a normal rhythm for the efficient pumping of blood. In addition to pacers, cardiac rhythm management systems also include, among other things, pacer/defibrillators that combine the functions of pacers and defibrillators, drug delivery devices, and any other implantable or external systems or devices for diagnosing or treating cardiac arrhythmias.
One problem faced by cardiac rhythm management systems is in delivering the high energy defibrillation shock. In one example, a transformer-coupled dc-to-dc voltage converter transforms a battery voltage (e.g., battery voltages approximately between 1.5 Volts and 6.5 Volts) up to a high defibrillation voltage (e.g., defibrillation voltages up to approximately 1000 Volts). The energy associated with this high defibrillation voltage is typically stored on a storage capacitor. A defibrillation energy delivery circuit delivers the defibrillation energy from the storage capacitor to defibrillation leadwires and defibrillation electrodes associated with the heart. Upon receiving this defibrillation energy via the defibrillation electrodes, the heart resumes normal rhythms if the defibrillation therapy is successful.
The defibrillation energy delivery circuit typically includes numerous discrete electronic components that must be capable of withstanding the large voltages associated with the defibrillation energy being delivered. These numerous discrete electronic components in the defibrillation energy delivery circuit occupy considerable space in the implantable cardiac rhythm management device. In order to improve patient comfort and aesthetics, however, the implantable cardiac rhythm management device should be small sized. Thus, a need exists for, among other things, reducing the size and/or cost of the defibrillation energy delivery circuit.